Clathrin-coated vesicles and membranes have been implicated in a variety of intracellular transport processes in eukaryotic cells. The precise funciton of clathrin in cell growth and protein transport will be addressed by evaluation of yeast mutants defective in production of clathrin heavy and light subunits. Polyclonal antiserum was used to identify a clone of the yeast heavy chain gene (CHC1) expressed in a Lambdagtll genomic library. The identity of this insert was confirmed by hybrid-selected translation of heavy chain mRNA followed by immune precipitation. A fragment of the insert was then used to generate deletions of the chromosomal CHC1 locus. Surprisingly, viable deletion mutant strains were produced which have no detectable immunoreactive heavy chain, and which produce vesicles that also lack clathrin light chain. Although clathrin deletion mutant cells grow slowly, secretion of the glycoprotein invertase is nearly normal. Deletion mutant cells will now be examined for the rate and fidelity of transport and sorting of proteins to various destinations. Structural analogues of clathrin will be sought using sensitive nucleic acid hybridization procedures and by examining alternative associations formed with clathrin light chain in heavy chain deletion mutant cells. Cloning and deletion analysis of the light chain gene will be used to detect any independent role that light chains may play in protein transport.